The present invention relates to a revolution speed ratio control system for a continuously or infinitely speed variable transmission which comprises a driving pulley, a driven pulley and an endless belt wound on both the pulleys and is used for a machine tool, a conveyor, a vehicle or the like.
A continuously variable transmission for a machine tool, a conveyor, a vehicle or the like is well known. In the continuously variable transmission, a driving pulley is provided on a driving shaft, a driven pulley is provided on a driven shaft, and an endless belt is wound on both the pulleys. Each of the pulleys comprises a fixed member secured to the shaft and having a conical surface whose vertex is located on the axis of gyration of the shaft, a movable member slidable in the direction of the axis of gyration of the shaft and having a conical surface whose vertex is located on the axis and which faces the conical surface of the fixed member, and a hydraulic cylinder secured to the shaft so as to push the movable member toward the fixed member. The endless belt is pinched between the conical surfaces of the fixed and the movable members to transmit motive power between both the pulleys. The ratio of the revolution speed of the driven shaft to that of the driving shaft can be altered in a continuous or infinite manner by changing the radial position of the endless belt on each of the pulleys.
The revolution speed ratio of the continuously variable transmission depends on the ratio of the running diameter of the endless belt on the driving pulley to that of the endless belt on the driven pulley. The speed ratio also depends on the clamp force (hereinafter referred to as the thrust to the belt) of the fixed and movable members of the driving pulley, which is to be applied to the belt. If an input torque and an RPM of the driving shaft are kept constant, the speed ratio can be kept constant where the thrusts of the driving and the driven pulley balance with each other. A hydraulic control circuit provided for the continuously variable transmission comprises a pump, a pressure control valve which functions so that the pressure of oil discharged from the pump and fed to a main oil passage is controlled to a prescribed level (referred to as line pressure), a revolution speed ratio control valve which is provided between the main oil passage and a first oil passage to the hydraulic cylinder or drains the oil from the cylinder to alter the pressure in the cylinder, a second oil passage for directly feeding the oil of the line pressure from the main oil passage to the driven pulley, and a revolution speed ratio feedback means in which the position of the movable member of the driven pulley is mechanically detected and the quantity of the displacement of the spring of the pressure control valve is regulated by a linkage to provide the line pressure with a component corresponding to the speed ratio. The speed ratio control valve is regulated by a control signal supplied from an electronic control circuit, to control the speed ratio.
The oil pressure applied to the hydraulic cylinder of the driving pulley is altered to break the balance of the thrusts of the driving and the driven pulleys to change the output speed of the conventional continuously variable transmission. The hydraulic cylinder of the driven pulley is directly connected to the main oil passage, while a balance breaking thrust is caused exclusively by the driving pulley.
When the oil pressure applied to the hydraulic cylinder of the driving pulley is increased as the thrusts of the driving and the driven pulleys to the endless belt wound thereon are in balance with each other, the continuously variable transmission is put into a higher output speed position. When the oil pressure is decreased as the thrusts are in balance with each other, the transmission is put into a lower output speed position. Since the transmission of motive power between each pulley and the endless belt is performed by the frictional force between the pulley and the belt, the thrust acting to the belt is determined by the relationship between the area and pressure of the contact of the pulley and the belt. For that reason, in order to heighten the motive power transmission efficiency of the stepless transmission, the contact surface pressure between each pulley and the belt needs to be kept at a prescribed level.
When the ratio of the revolution speed of the driven pulley to that of the driving pulley increases, the area of contact of the driven pulley and the endless belt decreases. At that time, the line pressure in the main oil passage is decreased by the speed ratio feedback means so that the thrust of the driven pulley to the belt is reduced. For that reason, a thrust control range for obtaining a desired speed ratio by controlling the thrust of the driving pulley through the control of the pressure in the hydraulic cylinder of the driving pulley is made narrow. As a result, it takes a long speed change time for a conventional control system to obtain a predetermined speed ratio. Therefore, the speed response of the conventional continuously variable transmission is not always good. The line pressure could be heightened in order to increase the thrust of the driven pulley in a higher output speed position of the continuously variable transmission to improve the speed responsibility. In that case, however, the contact surface pressure between the endless belt and the driven pulley in the higher output speed position of the transmission could tend to be much higher than that between the belt and the driving pulley because of the relationship between the running diameters of the belt on the pulleys so as to accelerate the fatigue and wear of the belt and the driven pulley to shorten their service lives.
In the prior art, a differential cylinder system is adopted in order that the absolute value of the difference between the thrusts of the driving and the driven pulleys in the higher output speed position of the continuously variable transmission is equal to that in the lower output speed position thereof. In the system, the cross-sectional area of the piston of the hydraulic cylinder of the driving pulley is made twice as much as that of the piston of the hydraulic cylinder of the driven pulley. When the contact surface pressure between the endless belt and each pulley is higher than a limit, a high resistance is caused to the separation of the belt from the pulley to drop the efficiency of the continuously variable transmission. When the contact surface pressure is lower than a limit, the frictional force between the endless belt and the pulley is too low to transmit prescribed motive power, namely, the belt slips relative to the pulley. Since the transmission of motive power depends on both the frictional force between the endless belt and each pulley or the oil pressure applied to the hydraulic cylinder of the pulley and the diameter of the belt running on the pulley and the speed ratio (running diameter ratio) depends on the difference between the thrusts of the pulleys, the cross-sectional area of the hydraulic cylinder of the driving pulley needs to be made about twice as much as that of the hydraulic cylinder of the driven pulley if the difference between the thrusts of the pulleys is to be made effective with the limited cross-sectional areas of the hydraulic cylinders. This hinders the reduction in the size and weight of the continuously variable transmission.